Energy Storage Technologies

Pumped Hydro Storage (PHS) or Pumped Generation Storage (PGS)

One of the larger scale technologies is PHS. It is not a new technology either, and has been around for over a century. It is used for bulk storage, with system sizes normally varying between 300 and 1,800 MW (Baxter, 2006).  PHS can be used to manage large portions of capacity from day to day or even for seasonal fluctuations.

A PHS system works by moving water between two reservoirs. Generally a PHS plant consists of two reservoirs, one at a higher elevation and another at a lower.  During off-peak hours, or when there is excess energy from renewables being produced, water is pumped from the lower elevation to the higher.  During peak hours this higher reservoir is then released back to the lower reservoir, passing through a turbine on the way (and generating electricity as it does so). 

Compressed Air Energy Storage (CAES)

With a CAES system, air is pressurized into a cavern during off-peak hours, generally to power a gas turbine during peak hours.  This technology allows for three times more electricity to be produced from a given unit of natural gas, and comes in units that exceed 100MW (Baxter, 2006), and making it suitable for bulk storage applications. Wind power can play a role here, in storing off-peak or excess wind energy from one or more wind farms. 

Battery Technologies

  • Flow Batteries - Flow battery technology features a reversible chemical reaction between two electrolytes is used to store and release energy, and these electrolytes vary depending on the flow battery system in question.  Flow battery types include zinc bromine, vanadium redox, polysulfide bromide and cerium zinc. A key feature of flow batteries is that the power and energy ratings are independent of each other, i.e. the number of cells gives the power rating (MW) and the storage tank size gives the energy rating (MWh). So it is a modular design, and they are flexible and scalable.  The two electrolytes are in separate tanks and during operation they flow from these to the cell stack to react there across a membrane, then the spent electrolytes are returned to the tanks.  When you recharge the flow battery, this reaction is reversed.  They have a role in increasing power quality and supporting transmission.
  • Sodium Sulfur Battery
  • Lead-Acid Battery
  • Nickel Cadmium Battery

Flywheels

Flywheels level power flow that comes in and out of a spinning mechanical device (like a turbine). Flywheels are a storage device in that they store the energy as kinetic energy through accelerating a rotor very fast. The rotor slows down as the energy is released, a reversal of the charging process. Flywheels are ideal for applications that need frequent and deep discharges because energy is mechanically stored, not chemically (less damage in discharging). Flywheels are used for enhancing power quality: through providing frequency regulation, uninterruptible supply, and regenerative energy services. For frequency regulation it smooths moment to moment fluctuations and is ideal for small or off-grid uses.

Flywheels also have a role in increasing the efficiency of transportation. Since flywheels can sustain repetitive motion, they are well suited for light-rail trains that need short bursts of power and frequently stop.   

Hydrogen

Hydrogen can be derived from either natural gas or water. With Natural gas, the carbon molecules are stripped off the methane molecule, and the hydrogen is captured and stored. The other process is through passing an electrical current across water (electrolysis) to separate the water molecule into oxygen and hydrogen in a hydrolyzer. The hydrogen is captured and stored for later use, and it will be used in fuel cells or burned directly.